This invention relates to annunciator monitor circuits of the type used to identify malfunctioning status switches in a protection loop.
Annunciator monitor circuits are known which are used in conjunction with protection loop devices having a plurality of serially connected, normally closed status switches to monitor a physical condition. For example, when used in a burglar alarm environment, each status switch is typically physically connected to a point of entry into a building or room and the switches are coupled in a series loop connected to a source of electrical power and an alarm unit, such as a bell, an emergency telephone number dialing device, or the like. When the structure is compromised by an intruder, the status switch associated to the particular point of entry opens, thereby opening the series loop and causing the alarm to be triggered.
In alarm control units of the above type, status switch contact monitoring devices have been employed in the past in order to provide an indication of the integrity of the protection loop including the status switches. In some early systems, a single test lamp is merely inserted in series with the status switches, and the integrity of the entire loop including the status switches is indicated by illumination of the test lamp when the annunciator is activated by the application of electrical power. More recently developed monitoring systems have employed individual test lamps, such as light emitting diodes, positioned at each of the status switch locations to enable a visual check of the integrity of each individual status switch at its particular location. Still other contact monitoring systems have employed a digital read out device having a plurality of priority encoded input terminals each coupled to a different one of the status switches for providing a numerical indication of the integrity of each status switch. Each of the above types of status switch monitoring circuits suffers from the disadvantage of incompatibility with present day solid state alarm control units, which are designed to provide high sensitivity to changing input status signals and relatively low power consumption when activated. More particularly, the sensitivity of modern solid state alarm control circuits is so high that the insertion across the status switches of a contact monitoring device capable of drawing even a few milliamperes of current from the protection loop will compromise or defeat the alarm unit by providing a current path around the switch through its own internal resistance.
A further drawback to the single test lamp type of contact monitoring system, and also the multiple test lamp, switch location mounted type of contact monitoring system, is the difficulty in quickly verifying the integrity of the system and isolating a fault location. More particularly, in the former, single test lamp type, each status switch must be individually tested to determine the source of a fault; while in the latter type of system each point of entry of the protective structure must be visually inspected. Since a large percentage of service calls made by service technicians in the alarm trade usually results in the location of a single malfunctioning switch, or a door or window inadvertently left open, annunciator systems having status switch monitoring circuits of these two types are costly to maintain.